Crikey, it's been five years or so since I was on the Broads in a hire craft but have never paid to go under the bridge at Potter Heigham.
The pubs will be fine with flip flops, trainers, wellington boots.... Frankly, they don't care what you wear.
I have had fantastic meals in beautiful restaurants in relatively casual clothes. The businesses know their client base.
There is SOME hanging space but not loads.
My best tip to you is regarding fuel. Whoever you are hiring the boat from will send you off with a full tank, and will fill it up on your return and charge you the difference.
EVERY boat hire place charges well above the actual cost - the difference is pounds, not pennies. There are diesel stations broadside - take note and fill up as near to your return to base as possible. Incidentally, the broadside pumps supply exactly the same fuel as the boat hire pumps, so don't be bullied into believing the faff about inferior quality fuel. It really gets my goat. You pay top whack to hire the boat, you don't need to be ripped off for fuel as well.
I quite envy you your holiday. It is my idea of heaven.